AaSlt2 Is Required for Vegetative Growth, Stress Adaption, Infection Structure Formation, and Virulence in Alternaria alternata

Slt2 is an important component of the Slt2-MAPK pathway and plays critical regulatory roles in growth, cell wall integrity, melanin biosynthesis, and pathogenicity of plant fungi. <i>AaSlt2</i>, an ortholog of the <i>Saccharomyces cerevisiae Slt2</i> gene, was identified from <i>A. alternata</i> in this study, and its function was clarified by knockout of the gene. The Δ<i>AaSlt2</i> strain of <i>A. alternata</i> was found to be defective in spore morphology, vegetative growth, and sporulation. Analysis of gene expression showed that expression of the <i>AaSlt2</i> gene was significantly up-regulated during infection structure formation of <i>A. alternata</i> on hydrophobic and pear wax extract-coated surfaces. Further tests on onion epidermis confirmed that spore germination was reduced in the Δ<i>AaSlt2</i> strain, together with decreased formation of appressorium and infection hyphae. Moreover, the Δ<i>AaSlt2</i> strain was sensitive to cell wall inhibitors, and showed significantly reduced virulence on pear fruit. Furthermore, cell wall degradation enzyme (CWDE) activities, melanin accumulation, and toxin biosynthesis were significantly lower in the Δ<i>AaSlt2</i> strain. Overall, the findings demonstrate the critical involvement of <i>AaSlt2</i> in growth regulation, stress adaptation, infection structure formation, and virulence in <i>A. alternata</i>.